1. Field of the Invention
This invention relates to control systems for constant speed, constant frequency, induction motors, and more particularly to control systems designed to produce energy efficient motor operation.
2. Description of the Prior Art
Various techniques have been employed in the past to control the speed of the torque output of a constant frequency induction motor using impedance magnitude or impedance angle as a feedback signal. A speed controller, exemplified by U.S. Pat. No. 3,441,823 to Schlabach, is illustrated in the simplified single phase equivalent circuit shown in FIG. 1. In this circuit the line voltage is represented by voltage source 2, and the motor reactance by primary leakage reactance 4, secondary leakage reactance 6, and magnetizing reactance 8. The rotor resistance R as modified by the slip factor S (slip being the difference between synchronous and actual speed divided by the synchronous speed) is represented by variable resistance 10. An AC phase delay controller circuit 12, consisting of a pair of anti-parallel SCRs 13 and 14, responds to the difference between a speed command signal 15 and a speed feedback signal 16, which is derived from a motor impedance magnitude sensor 17, to regulate the portion of each half-cycle of the line voltage source 2 during which a switching device is conductive to energize the motor. The effective voltage across the motor windings depends on the duration of the switch closure and consequent motor energization; the effective motor voltage increases as the motor is energized earlier in each half-cycle, and decreases as the motor is energized later in each half-cycle. By controlling the period of motor energization in response to the impedance derived speed feedback signal, the circuit of FIG. 1 is able to achieve substantially constant speed operation over a wide range of speed command setpoints with varying load torque. High power operation is not practical, however, because of the rotor losses associated with the excessive slip which is present at speeds significantly less than synchronous speed.
An output torque controller based on impedance angle sensing, the concept of which is illustrated in FIG. 2, was designed to minimize motor losses at light load while operating at close to synchronous speed. This circuit, which is exemplified by U.S. Pat. No. 4,052,648 to Nola, adjusts the motor voltage to match the prevailing load torque requirement of the motor in response to the power factor present at the input to the AC phase delay controller. Power factor is defined as the ratio between (1) the input power and (2) the input voltage multiplied by the input current. For perfect sine wave signals, the power factor is equal to the cosine of the phase angle between the voltage and current signals. In the Nola approach, the motor voltage is varied as a function of the phase angle between the zero crossings of the sine wave input line voltage and the angle at which the motor current flow, which is discontinuous, ceases. Specifically, a commanded line power factor angle is applied as an input to a summing junction 18, which also receives as a negative input a signal over line 20 representing the actual line power factor angle. The output signal from summing junction 18 represents the difference between the commanded and actual line power factor angle, and is employed as an error signal to modify the motor voltage. This error signal is amplified in amplifier 22 and applied to an SCR gate delay generator circuit 24, which produces an output over line 26 consisting of a series of pulses that are delayed from the zero crossings of the line voltage 27 by an amount proportional to the negative of the error signal. Circuit 24 may be implemented by applying the line voltage to a ramp generator, comparing the ramp signals with the amplified error signal, and producing an output when the amplified error signal exceeds the ramp signal, or by other conventional means. The signal along line 26 is delivered to an AC phase delay controller circuit 28. This circuit corresponds to the AC phase delay controller circuit 12 of FIG. 1, and together with the SCR gate delay generator 24 regulates the period of time that the line voltage is connected to the motor terminals.
The output of AC phase delay controller 28 is applied to the motor stator 30. The resulting motor current is sensed by a current sensor circuit 32, which in turn is connected to a current signal comparator circuit 34 which produces an output over line 36 indicating the zero crossings of the motor current. At the same time, the line voltage is sensed by line voltage sensor circuit 38, the output of which is connected to a voltage signal comparator circuit 40 which provides an output on line 42 indicating the zero crossings of the line voltage. Lines 36 and 42 are connected to a phase detector circuit 44, which produces an output over line 46 indicating the phase angle difference between the line voltage and motor current. This signal is routed through a low pass filter 48, and thereafter applied as a DC signal over line 20 to summing junction 18.
While the circuit of FIG. 2 has been found to improve the energy efficiency of motors, various problems have been observed. First, there is a tendency for the motor to stall in response to a step or suddenly applied increase in load torque. If the loop gain is increased in an effort to solve the stalling problem, an element of positive feedback is noted, and the circuit can become instable. This problem has been found to be particularly severe with larger motors. A hunting problem at startup, rather than a smooth transition to synchronous speed, has also been encountered.